If you encrypt Windows system drive with BitLocker using USB startup key, you may get an error message such as this one:
BitLocker could not be enabled.
The data drive specified is not set to automatically unlock on the current computer and cannot be unlocked automatically.
C: was not encrypted.
In my case the problem was due to weird BIOS settings that seemed to have no effect when using Windows 10 but affected the boot and thus BitLocker too. The setting
Advanced - Peripheral Configuration - USB Host Controller - USB Devices
was set to None and the problem was solved when this setting was changed to All.
Basically the setting seemed to prevent the use of USB drive when booting. So if you experience similar BitLocker problems with any other BIOS, try to find a setting that controls the use of USB drives.